Plug-in hybrid electric vehicles (PHEV) may include hydrocarbon (HC) traps coupled in an air induction system of the engine to adsorb evaporative emissions in the engine intake. For example, fuel vapors may result from fuel sprayed into the intake manifold, fuel leaked from fuel injectors and/or fuel puddled in the intake. These fuel vapors, if not stored in the hydrocarbon trap, may flow out of the intake system during engine shutdown, increasing evaporative emissions.
Example hydrocarbon traps are shown by Moyer et al in US 2013/0228145 and Burke et al. in US 2006/0054142. The hydrocarbon traps are positioned in the intake system so that fuel vapors can be passively adsorbed in and released from the hydrocarbon trap. Specifically, during engine operation, intake manifold vacuum draws the fuel vapors into the engine and also causes airflow across the trap, increasing desorption. Desorbed vapors are subsequently combusted in the engine.
However the inventors herein have identified potential issues with such systems. The reduced engine operation time of PHEVs may limit the amount of intake manifold vacuum generated for hydrocarbon trap purging. As such, airflow across the trap may be limited, reducing passive desorption of fuel vapors. Incomplete purging of the hydrocarbon trap can result in an increase in intake evaporative emissions and degrade vehicle emissions performance.
The inventors herein have recognized the above issue and identified an approach to at least partly address the issue. In one example approach, a method for a hybrid vehicle with an engine including a HC trap in an intake of the engine is provided. The method comprises, during vehicle travel in an engine-off condition, increasing airflow through a HC trap and purging fuel vapors from the trap to a fuel system canister coupled to a fuel tank. In this way, a HC trap can be at least partially purged in an efficient way, even during conditions when the engine is not running.
For example, while operating the hybrid vehicle in an electric mode where wheel torque is provided by a system battery and an engine is maintained shutdown (e.g., at rest), HC trap purging may be enabled when there is sufficient vehicle motion-induced airflow. For example, once the vehicle speed exceeds a threshold, the intake throttle may be partially or fully opened to increase airflow through the intake system and past the HC trap. Concurrently, cylinder intake and exhaust valves may be closed (e.g., via cam timing adjustments), if not already closed, and a canister purge valve may be opened so as to route motion-inducted airflow containing released HCs from the HC trap to the fuel system. The purge fuel vapors desorbed from the HC trap can be routed into and through a fuel system canister so as to transport the stored HCs from the intake system to be stored in the fuel system. Intake airflow may also be further enhanced during the engine-off condition via adjustments to grill shutters and louvers, as well as by operating a cooling system fan.
In this way, intake fuel vapors stored in an air induction system HC trap may be opportunistically transferred to a fuel system canister with larger storage capacity for storage therein during engine-off conditions. By opening an intake throttle during vehicle travel in an electric mode, vehicle motion is advantageously used to generate airflow in a direction from ambient, to and along the hydrocarbon trap, and then to another storage location before finally venting to atmosphere. This enables the trap to be at least partially cleaned even while an engine is shutdown and no intake manifold vacuum is generated. By delivering the fuel vapors from the smaller capacity trap to a larger capacity fuel system canister, the canister may be used as a HC vapor “storage bank” so that the HC trap may be cleaned out more often until the engine is operated to clean out the canister (and the HC trap). This results in reduced HC breakthrough from the HC trap. Overall exhaust emissions and hybrid vehicle performance are thereby improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.